1. Technical Field
The present invention relates to a method of calibrating a humidity sensor.
2. Description of the Related Art
Humidity sensors are used to measure the relative humidity in open or closed volumes of gas, i.e., the quantity of humidity that is dissolved in the particular gas relative to the maximum quantity of humidity soluble in the gas at the given temperature.
Such measurements are familiar to, for example, the average consumer as measurements of relative atmospheric humidity for the determination of climatic conditions, but they are also common in the industrial field for determination of, for example, the relative atmospheric humidity in gas vessels under pressure.
Humidity sensors are often severely stressed by physical and chemical influences and their response characteristic may thereby be significantly altered with respect to the initial calibration. Recalibration is therefore necessary at certain intervals.
The related art (German Patent Application 3936138 A1, U.S. Pat. No. 6,073,480) includes calibration of a humidity sensor in that a first humidity measurement is carried out at a first temperature and then a second humidity measurement is carried out at a second temperature, the gas pressure remaining the same in both measurements. The measured temperature and humidity values permit computation of a correction value for the humidity measurement, since the mathematical relationship between temperature and actual relative humidity is known.
However, for the use of this technique it is necessary to place temperature sensors in the region of the humidity sensors, as well as to provide heating or cooling elements. In addition, such a method of calibration requires time for setting the particular measurement temperatures.
The physical and chemical influences result in a change or a drift in the characteristic curve of the sensor. The change results in an offset but not in a change of the slope of the characteristic curve. This is particularly true for small values of humidity, e.g., humidity values below 15 percent, and which is important when, for example, monitoring the remaining humidity in compressed air.
Accordingly, it would be desirable to provide a method of recalibrating a humidity sensor, which had initially been calibrated and then changed its characteristic curve due to physical and chemical influences during its operation, and a corresponding device. It may also be desirable to provide a sensor arrangement having a calibratable humidity sensor of the type mentioned at the beginning, wherein the calibration procedure is simplified and as few as possible auxiliary means are required for its performance.